A gold nanoparticle is a small particle of gold whose optical and surface chemistry depend on its geometry and surroundings. A dispersion of such particles can appear red or purple rather than resembling a piece of bulk gold.[ref]
What Is So Special about Nano? by the National Nanotechnology Coordination Office. Nanoscale gold and size-dependent colour
In 1857, Michael Faraday described ruby-coloured fluids containing finely divided gold. He compared the transmitted colour of these fluids with coloured glass and observed changes toward blue or violet under conditions that altered the dispersed particles. These observations established a striking connection between the physical state of gold and its interaction with light.[ref]
Experimental Relations of Gold and Other Metals to Light by Michael Faraday. Philosophical Transactions 147, pages 159–168, especially the ruby fluids and comparison with ruby glass
A modern explanation involves localized surface plasmon resonance: the collective response of conduction electrons to an incident electromagnetic field changes absorption and scattering. The resulting spectrum depends on particle shape and the surrounding dielectric, as well as size. Thus colour alone is not a universal measure of nanoparticle diameter.
Experiments on gold nanorings show how geometry changes the response: varying the ratio of ring thickness to radius shifts the localized surface plasmon resonance. This is evidence for control through particle shape rather than a change of chemical element.[ref]
Optical Properties of Gold Nanorings by Javier Aizpurua and others. Physical Review Letters 90 (2003), abstract: measured shifts and coupling of inner and outer surface modes
A gold nanoring is a ring-shaped gold nanostructure whose central opening and wall dimensions alter its response to light. Charge oscillations associated with its inner and outer surfaces couple, changing its localized surface plasmon resonance relative to that of a compact particle.[ref]
Optical Properties of Gold Nanorings by Javier Aizpurua and others. Abstract, measured resonance shifts and coupled surface modes
The 2003 experiments and modelling showed that varying the ring geometry shifts the optical resonance. Comparing rings with different wall-thickness-to-radius ratios thus tests a geometric mechanism while retaining the same elemental material. A measured colour or spectral peak must be interpreted together with geometry and the surrounding dielectric, not as a unique measure of overall particle diameter.[ref]
Optical Properties of Gold Nanorings by Javier Aizpurua and others. Geometry-dependent optical response
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